Closed cycle gas turbine systems wherein a gas is confined in a closed circuit and heated by fossil fuel to directly drive a turbine is old. It is also old to drive a turbine by steam generated in a secondary loop which is exposed in a heat exchanger to gas heated in a gas cooled nuclear reactor. A turbine driven directly by gas in a so-called closed gas turbine Brayton cycle utilizing as the heat source system a gas cooled nuclear reactor is desirable since such a system reduces the complexities called for by a secondary loop and would reduce the overall size of the system to the point where it could usefully drive automobiles, air craft and smaller water craft. A problem has existed, however, with all known systems, either in actual use or as discussed in technical papers, in the control of the gas supply from the heat source to the turbine. Heretofore transient control of gas supply to gas driven turbines has been by two primary methods: 1. by-pass control where a portion of the gas flow is by-passed around the turbine, and 2. inventory control, where the working pressure of the gas is adjusted to match a particular power level. Though both systems have advantages both also have disadvantages. By-pass control is relatively fast but has the disadvantage of low cycle efficiencies at significant by-pass flows. Inventory control requires high and low pressure reservoirs which take up valuable space, and both systems require additional apparatus all of which present additional points of failure as well as maintenance requirements calling for additional personnel with the required expertise to service the additional apparatus.
There is thus a need for better control of the gas flow to a turbine which eliminates the foregoing disadvantages and it is the broad object of this invention to provide a closed gas turbine system, and particularly a direct closed cycle gas turbine system (Brayton cycle) utilizing a gas cooled nuclear reactor as the heat source, and which includes, in accordance with the invention an improved gas flow control system never heretofore contemplated in the art of closed cycle gas turbine systems.